Low pass filter

ABSTRACT

When a plurality of RF signals having different frequency bands are output at the same time by carrier aggregation, a switch element allows parallel connection between two capacitance elements such that a low pass filter has a first cut-off frequency that is lower than the frequency of an intermodulation distortion signal generated by the carrier aggregation. When an RF signal of a frequency band is output, the switch element releases parallel connection between the two capacitance elements such that the low pass filter has a second cut-off frequency that is higher than the first cut-off frequency.

This application claims priority from Japanese Patent Application No.2017-002013 filed on Jan. 10, 2017. The content of this application isincorporated herein by reference in its entirety.

BACKGROUND

The present disclosure relates to a low pass filter. As a technique forimproving the communication speed in mobile communication terminals,carrier aggregation has been known in which radio frequency (RF) signalsare transmitted and received at the same time using a singlecommunication line formed by collection of a plurality of componentcarriers (CC) having different frequency bands. According to JapaneseUnexamined Patent Application Publication No. 2016-92699, it is reportedthat, based on uplink carrier aggregation, when a plurality of RFsignals of different frequency bands are amplified by a single poweramplifier, intermodulation distortion corresponding to distortioncharacteristics of the power amplifier occurs. An intermodulationdistortion signal serves as a spurious component of an RF signalradiated from an antenna, and provides interference to communication ina frequency band to which a CC belongs or an adjacent frequency band.For example, when a signal in a band of about 800 MHz and a signal in aband of about 1.7 GHz are amplified at the same time, a five-orderintermodulation distortion signal is generated in a band from about 1535MHz to about 1615 MHz. As described above, in an operation based oncarrier aggregation, generation of intermodulation distortion andharmonic distortion is a factor for a degradation in characteristics.

For example, as a method for reducing intermodulation distortiongenerated by uplink/downlink carrier aggregation, for example, a methodfor mounting a low pass filter for attenuating an intermodulationdistortion signal at a transmission path for an RF signal is considered.

However, such a low pass filter has a drawback in that unnecessaryinsertion loss occurs in a transmission path for an RF signal whenuplink/downlink carrier aggregation is not implemented.

BRIEF SUMMARY

Accordingly, the present disclosure solves the above drawback and toreduce insertion loss of a low pass filter.

According to embodiments of the present disclosure, a low pass filterthat inputs and outputs an RF signal includes (i) a first switchelement; (ii) a first capacitance element whose one end is connected toa signal line for the low pass filter and whose other end is connectedto ground of the low pass filter; and (iii) a second capacitance elementwhose one end is connected to the signal line and whose other end isconnected to the ground via the first switch element. When a pluralityof RF signals having different frequency bands are output at the sametime by carrier aggregation, the first switch element is turned on suchthat the low pass filter has a first cut-off frequency that is lowerthan a frequency of an intermodulation distortion signal generated bythe carrier aggregation, thereby the first capacitance element and thesecond capacitance element being connected in parallel between thesignal line and the ground. In contrast, when an RF signal of afrequency band is output, the first switch element is turned off suchthat the low pass filter has a second cut-off frequency that is higherthan the first cut-off frequency, thereby parallel connection betweenthe first capacitance element and the second capacitance element beingreleased.

According to embodiments of the present disclosure, insertion loss of alow pass filter may be reduced.

Other features, elements, and characteristics of the present disclosurewill become more apparent from the following detailed description ofembodiments of the present disclosure with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a circuit configuration of awireless communication circuit according to a first embodiment of thepresent disclosure;

FIG. 2 is a circuit diagram illustrating an example of a circuitconfiguration of a low pass filter according to a first embodiment ofthe present disclosure;

FIG. 3 illustrates simulation results representing frequencycharacteristics of the low pass filter according to the first embodimentof the present disclosure;

FIG. 4 is a circuit diagram illustrating an example of a circuitconfiguration of a low pass filter according to a second embodiment ofthe present disclosure;

FIG. 5 is a circuit diagram illustrating an example of a circuitconfiguration of a low pass filter according to a third embodiment ofthe present disclosure;

FIG. 6 is a circuit diagram illustrating an example of a circuitconfiguration of a low pass filter according to a fourth embodiment ofthe present disclosure;

FIG. 7 is a circuit diagram illustrating an example of a circuitconfiguration of a low pass filter according to a fifth embodiment ofthe present disclosure;

FIG. 8 is a circuit diagram illustrating an example of a circuitconfiguration of a low pass filter according to a sixth embodiment ofthe present disclosure;

FIG. 9 is a circuit diagram illustrating an example of a circuitconfiguration of a low pass filter according to a reference example; and

FIG. 10 is a circuit diagram illustrating an example of a circuitconfiguration of a low pass filter according to a seventh embodiment ofthe present disclosure.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described with referenceto drawings. The same reference signs will represent the same circuitelements, and redundant explanation will be omitted. In each of theembodiments described below, a case where an uplink carrier aggregationoperation is performed will be described as an example. However, thepresent disclosure may also be applied to a case where a downlinkcarrier aggregation operation is performed.

FIG. 1 is a block diagram illustrating a circuit configuration of awireless communication circuit 10 according to a first embodiment of thepresent disclosure. The wireless communication circuit 10 is a module,in a mobile communication apparatus such as a cellular phone, fortransmitting and receiving RF signals to and from a base station. Thewireless communication circuit 10 is also capable of, by carrieraggregation, transmitting and receiving a plurality of RF signals havingdifferent frequency bands at the same time. The wireless communicationcircuit 10 includes a baseband integrated circuit (IC) 20, a radiofrequency integrated circuit (RFIC) 30, a high frequency module 40, andan antenna 90. The high frequency module 40 includes a transmissionmodule 50, a reception module 60, an antenna switch (ANTSW) 70, and alow pass filter (LPF) 80.

The baseband IC 20 performs encoding and modulation of communicationinformation in accordance with a predetermined communication method, andgenerates a baseband signal by digital signal processing. The RFIC 30generates an RF signal, as a transmission signal, by modulating carrierwaves in accordance with information superimposed on a baseband signal.The transmission module 50 includes a power amplifier 51 and atransmission filter 52, and performs filtering corresponding to afrequency band of a transmission signal and power amplification of thetransmission signal. The power amplifier 51 includes one or moretransistor elements.

The antenna switch 70 includes a node 71 connected to the transmissionmodule 50, a node 72 connected to the reception module 60, and a node 73connected to the antenna 90. The low pass filter 80 which inputs andoutputs an RF signal output from the power amplifier 51 is mounted at asignal path between the antenna switch 70 and the antenna 90. Theantenna switch 70 selectively establishes a signal path for atransmission signal between the nodes 71 and 73. Accordingly, the lowpass filter 80 is connected to output of the power amplifier 51. When aplurality of RF signals having different frequency bands are output fromthe power amplifier 51 at the same time by carrier aggregation, the lowpass filter 80 attenuates an intermodulation distortion signal generatedby the carrier aggregation.

The antenna switch 70 selectively establishes a signal path for an RFsignal, as a reception signal received from a base station via theantenna 90, between the nodes 72 and 73. Accordingly, the low passfilter 80 is connected to input of a low-noise amplifier 61. Thereception module 60 includes the low-noise amplifier 61 and a receptionfilter 62, and performs filtering corresponding to a frequency band of areception signal and low-noise amplification of the reception signal.The RFIC 30 converts a reception signal into a baseband signal. Thebaseband IC 20 demodulates and decodes a baseband signal, and extractscommunication information.

FIG. 2 is a circuit diagram illustrating an example of a circuitconfiguration of the low pass filter 80 according to the firstembodiment of the present disclosure. The low pass filter 80 attenuatesa high frequency component of an RF signal input to an input/output node81, and outputs the resultant signal from an input/output node 82. Theinput/output node 81 is connected to the node 73 of the antenna switch70, and the input/output node 82 is connected to the antenna 90. A lineconnecting the input/output node 81 with the input/output node 82 isreferred to as a signal line 87. The low pass filter 80 is the oneobtained by partially changing a circuit of a known low pass filterhaving elliptic function characteristics, and includes a parallelresonance circuit including a capacitance element C1 and an inductorelement L1. With this parallel resonance, sharp attenuationcharacteristics may be achieved near a cut-off frequency. The parallelresonance circuit is connected in series with the signal line 87. One oftwo nodes as points at which the capacitance element C1 and the inductorelement L1 are connected in parallel is referred to as a node 83, andthe other node is referred to as a node 84. An inductor element L2 isconnected between the input/output node 81 and the node 83. An inductorelement L3 and a capacitance element C21 are connected in series betweenthe node 83 and the ground (that is, between the signal line 87 and theground). As described above, one end of the capacitance element C21 isconnected to the signal line 87, and the other end of the capacitanceelement C21 is connected to the ground. A point at which the inductorelement L3 and the capacitance element C21 are connected is referred toas a node 85. A capacitance element C22 and a switch element SW1 areconnected in series between the node 85 and the ground (that is, betweenthe signal line 87 and the ground). As described above, one end of thecapacitance element C22 is connected to the signal line 87, and theother end of the capacitance element C22 is connected to the ground viathe switch element SW1. An inductor element L4 and a capacitance elementC31 are connected in series between the node 84 and the ground (that is,between the signal line 87 and the ground). As described above, one endof the capacitance element C31 is connected to the signal line 87, andthe other end of the capacitance element C31 is connected to the ground.A point at which the inductor element L4 and the capacitance element C31are connected is referred to as a node 86. A capacitance element C32 anda switch element SW2 are connected in series between the node 86 and theground (that is, between the signal line 87 and the ground). Asdescribed above, one end of the capacitance element C32 is connected tothe signal line 87, and the other end of the capacitance element C32 isconnected to the ground via the switch element SW2.

Each of the switch elements SW1 and SW2 is a semiconductor switch, suchas a field effect transistor, which is called a single pole single throw(SPST) switch. When the switch element SW1 is turned on, the capacitanceelements C21 and C22 are connected in parallel between the signal line87 and the ground. In contrast, when the switch element SW1 is turnedoff, parallel connection between the capacitance elements C21 and C22 isreleased. As described above, the switch element SW1 selectively allowsparallel connection between the capacitance elements C21 and C22.Hereinafter, in the case where the capacitance elements C21 and C22 aredistinguished from each other, the capacitance element C21 will bereferred to as a first capacitance element, and the capacitance elementC22 will be referred to as a second capacitance element. Similarly, whenthe switch element SW2 is turned on, the capacitance elements C31 andC32 are connected in parallel between the signal line 87 and the ground.In contrast, when the switch element SW2 is turned off, parallelconnection between the capacitance elements C31 and C32 is released. Asdescribed above, the switch element SW2 selectively allows parallelconnection between the capacitance elements C31 and C32. Hereinafter, inthe case where the capacitance elements C31 and C32 are distinguishedfrom each other, the capacitance element C31 will be referred to as afirst capacitance element, and the capacitance element C32 will bereferred to as a second capacitance element. In accordance with ON/OFFoperations of the switch elements SW1 and SW2, frequency characteristics(attenuation characteristics) of the low pass filter 80 may be changed.More particularly, when the switch elements SW1 and SW2 are turned on,the cut-off frequency of the low pass filter 80 may be reduced. Incontrast, when the switch elements SW1 and SW2 are turned off, thecut-off frequency of the low pass filter 80 may be increased. The switchelements SW1 and SW2 perform ON/OFF operations in response to, forexample, a control signal from the baseband IC 20 or the RFIC 30.

The low pass filter 80 is designed to operate in one operation modeselected from two operation modes. An operation mode that is selectedwhen a plurality of RF signals having different frequency bands areoutput from the power amplifier 51 at the same time by carrieraggregation is referred to as a first operation mode. In the firstoperation mode, the switch elements SW1 and SW2 are in an ON state suchthat the low pass filter 80 has a cut-off frequency that is lower thanthe frequency of an intermodulation distortion signal generated bycarrier aggregation. Accordingly, the low pass filter 80 may attenuatethe intermodulation distortion signal. For the convenience ofexplanation, the cut-off frequency of the low pass filter 80 in thefirst operation mode is referred to as a first cut-off frequency. Anoperation mode that is selected when an RF signal of one frequency bandis output from the power amplifier 51 (that is, uplink carrieraggregation is not performed) is referred to as a second operation mode.In the second operation mode, the switch elements SW1 and SW2 are in anOFF state such that the low pass filter 80 has a cut-off frequency thatis higher than the first cut-off frequency. Accordingly, the pass bandof the low pass filter 80 extends from a low frequency band to a wideregion including a low frequency band and a high frequency band.Therefore, insertion loss by the low pass filter 80 occurring in an RFsignal output from the power amplifier 51 may be reduced. In the secondoperation mode, the low pass filter 80 does not need to attenuate anintermodulation distortion signal generated by carrier aggregation.Therefore, it is desirable that the cut-off frequency of the low passfilter 80 is as high as possible. For the convenience of explanation,the cut-off frequency of the low pass filter 80 in the second operationmode is referred to as a second cut-off frequency.

When the switch elements SW1 and SW2 are in the ON state, the switchelements SW1 and SW2 behave as equivalent to resistance elements. Whenthe switch elements SW1 and SW2 are in the OFF state, the switchelements SW1 and SW2 behave as equivalent to capacitance elements. Inthe light of the above, the first cut-off frequency of the low passfilter 80 is determined based on element values (capacitance values andinductance values) of the individual circuit elements (the capacitanceelements C1, C21, C22, C31, and 32 and the inductor elements L1, L2, L3,and L4) forming the low pass filter 80 and ON resistances of the switchelements SW1 and SW2. Similarly, the second cut-off frequency of the lowpass filter 80 is determined based on element values (capacitance valuesand inductance values) of the individual circuit elements (thecapacitance elements C1, C21, C22, C31, and C32 and the inductorelements L1, L2, L3, and L4) forming the low pass filter 80 and OFFcapacitances of the switch elements SW1 and SW2. An ON resistancerepresents a resistance value of a switch element in the ON state. AnOFF capacitance represents a capacitance value of a switch element inthe OFF state.

FIG. 3 illustrates simulation results representing frequencycharacteristics of the low pass filter 80. Reference numeral 31represents frequency characteristics of the low pass filter 80 in thefirst operation mode. Reference sign 32 represents frequencycharacteristics of the low pass filter 80 in the second operation mode.As is clear from FIG. 3, when the operation mode of the low pass filter80 is switched from the first operation mode to the second operationmode, attenuation characteristics becomes moderate, the pass band of thelow pass filter 80 extends from a low frequency band to a wide regionincluding a low frequency band and a high frequency band. Accordingly,in the second operation mode, insertion loss by the low pass filter 80occurring in an RF signal output from the power amplifier 51 may bereduced.

As described above, with the low pass filter 80 according to the firstembodiment, in accordance with ON/OFF operations of the switch elementsSW1 and SW2, an intermodulation distortion signal may be attenuated inthe first operation mode, and the insertion loss of the low pass filter80 may be reduced in the second operation mode. Furthermore, bydetermining the first cut-off frequency of the low pass filter 80 basedon the element values of the individual circuit elements forming the lowpass filter 80 and the ON resistances of the switch elements SW1 andSW2, design accuracy may be increased. Similarly, by determining thesecond cut-off frequency of the low pass filter 80 based on the elementvalues of the individual circuit elements forming the low pass filter 80and the OFF capacitances of the switch elements SW1 and SW2, designaccuracy may be increased.

The low pass filter 80 is not necessarily mounted at a signal pathbetween the antenna switch 70 and the antenna 90. For example, the lowpass filter 80 may be incorporated inside the antenna switch 70.Alternatively, the low pass filter 80 may be incorporated inside afront-end module of the wireless communication circuit 10. For example,a front-end module is a module implemented by the antenna switch 70, thetransmission filter 52, and the reception filter 62.

Furthermore, the low pass filter 80 does not necessarily have ellipticfunction characteristics. For example, the low pass filter 80 may haveChebyshev characteristics, Butterworth characteristics, Besselcharacteristics, or the like.

Next, a circuit configuration of a low pass filter 100 according to asecond embodiment of the present disclosure will be described withreference to FIG. 4, focusing on differences from the low pass filter 80according to the first embodiment. The low pass filter 100 has a circuitconfiguration in which the switch elements SW1 and SW2 of the low passfilter 80 is replaced by a switch element SW3 and the inductor elementsL2, L3, and L4 are eliminated from the low pass filter 80. In the secondembodiment, the switch element SW3 is a semiconductor switch, such as afield effect transistor, which is called an SPST switch, and performsON/OFF operations in response to, for example, a control signal from thebaseband IC 20 or the RFIC 30.

In the first operation mode, the switch element SW3 is in an ON statesuch that the capacitance elements C21 and C22 are connected in paralleland the capacitance elements C31 and C32 are connected in parallel.Accordingly, the low pass filter 100 has a first cut-off frequency thatis lower than the frequency of an intermodulation distortion signalgenerated by carrier aggregation. Thus, the low pass filter 100 mayattenuate the intermodulation distortion signal. In the second operationmode, the switch element SW3 is in an OFF state such that parallelconnection between the capacitance elements C21 and C22 is released andparallel connection between the capacitance elements C31 and C32 isreleased. Accordingly, the low pass filter 100 has a second cut-offfrequency that is higher than the first cut-off frequency. The pass bandof the low pass filter 100 extends from a low frequency band to a wideregion including a low frequency band and a high frequency band, andtherefore, may reduce insertion loss by the low pass filter 100occurring in an RF signal output from the power amplifier 51.

Next, a circuit configuration of a low pass filter 110 according to athird embodiment of the present disclosure will be described withreference to FIG. 5, focusing on differences from the low pass filter100 according to the second embodiment. The low pass filter 110 has acircuit configuration in which the capacitance element C32 is eliminatedfrom the low pass filter 100. Operation of the low pass filter 110 andoperation effects of the low pass filter 110 are similar to those of thelow pass filter 100.

Next, a circuit configuration of a low pass filter 120 according to afourth embodiment of the present disclosure will be described withreference to FIG. 6, focusing on differences from the low pass filter100 according to the second embodiment. The low pass filter 120 has acircuit configuration in which the inductor element L2 functioning as amatching circuit is added to the low pass filter 100. Operation of thelow pass filter 120 and operation effects of the low pass filter 120 aresimilar to those of the low pass filter 100.

Next, a circuit configuration of a low pass filter 130 according to afifth embodiment of the present disclosure will be described withreference to FIG. 7, focusing on differences from the low pass filter100 according to the second embodiment. The low pass filter 130 has acircuit configuration in which a parallel resonance circuit including acapacitance element C4 and the inductor element L4 is added to the lowpass filter 100. Operation of the low pass filter 130 and operationeffects of the low pass filter 130 are similar to those of the low passfilter 100.

Next, a circuit configuration of a low pass filter 140 according to asixth embodiment of the present disclosure will be described withreference to FIG. 8, focusing on differences from the low pass filter100 according to the second embodiment. The low pass filter 140 has acircuit configuration in which the capacitance element C1 of the lowpass filter 100 is replaced by a switch element SW4. The switch elementSW4 is a semiconductor switch, such as a field effect transistor, whichis called an SPST switch, and performs ON/OFF operations in response to,for example, a control signal from the baseband IC 20 or the RFIC 30.The switch element SW4 is connected in parallel with the inductorelement L1. The inductor element L1 is connected in series with a signalline for the low pass filter 140.

In the first operation mode, the switch element SW3 is in an ON stateand the switch element SW4 is in an OFF state such that the low passfilter 140 has a first cut-off frequency that is lower than thefrequency of an intermodulation distortion signal generated by carrieraggregation. When the switch element SW4 is in the OFF state, the switchelement SW4 behaves as equivalent to a capacitance element, and theswitch element SW4 and the inductor element L1 form a parallel resonancecircuit. With sharp attenuation characteristics near the first cut-offfrequency by parallel resonance, the low pass filter 140 may attenuatean intermodulation distortion signal. In the second operation mode, theswitch element SW3 is in the OFF state and the switch element SW4 is inthe ON state such that the low pass filter 140 has a second cut-offfrequency that is higher than the first cut-off frequency. When theswitch element SW4 is in the ON state, the switch element SW4 behaves asequivalent to a resistance element, and the switch element SW4 and theinductor element L1 do not form a parallel resonance circuit.Accordingly, the attenuation characteristics of the low pass filter 140become moderate, and the pass band of the low pass filter 140 extendsfrom a low frequency band to a wide region including a low frequencyband and a high frequency band. Therefore, insertion loss by the lowpass filter 140 occurring in an RF signal output from the poweramplifier 51 may be reduced.

The first cut-off frequency of the low pass filter 140 is determinedbased on element values (capacitance values and inductance values) ofthe individual circuit elements (the capacitance elements C21, C22, C31,and C32 and the inductor element L1) forming the low pass filter 140, anON resistance of the switch element SW3, and an OFF capacitance of theswitch element SW4. The second cut-off frequency of the low pass filter140 is determined based on element values (capacitance values andinductor values) of the individual circuit elements (the capacitanceelements C21, C22, C31, and C32 and the inductor element L1) forming thelow pass filter 140, an OFF capacitance of the switch element SW3, andan ON resistance of the switch element SW4.

As described above, with the low pass filter 140 according to the sixthembodiment, in accordance with ON/OFF operations of the switch elementsSW3 and SW4, an intermodulation distortion signal may be attenuated inthe first operation mode, and the insertion loss of the low pass filter140 may be reduced in the second operation mode. Furthermore, bydetermining the first cut-off frequency of the low pass filter 140 basedon the element values of the individual circuit elements forming the lowpass filter 140, the ON resistance of the switch element SW3, and theOFF capacitance of the switch element SW4, design accuracy may beincreased. Similarly, by determining the second cut-off frequency of thelow pass filter 140 based on the element values of the individualcircuit elements forming the low pass filter 140, the OFF capacitance ofthe switch element SW3, and the ON resistance of the switch element SW4,design accuracy may be increased.

A function of the switch element SW4 is different from functions of theswitch elements SW1, SW2, and SW3. Therefore, the switch elements SW1,SW2, and SW3 may be referred to as first switch elements, and the switchelement SW4 may be referred to as a second switch element.

Next, a circuit configuration of a low pass filter 150 according to areference example will be described with reference to FIG. 9, focusingon differences from the low pass filter 140 according to the sixthembodiment. The low pass filter 150 has a circuit configuration in whichthe capacitance elements C22 and C32 and the switch element SW3 areeliminated from the low pass filter 140.

In the first operation mode, the switch element SW4 is in an OFF statesuch that the low pass filter 150 has a first cut-off frequency that islower than the frequency of an intermodulation distortion signalgenerated by carrier aggregation. When the switch element SW4 is in theOFF state, the switch element SW4 behaves as equivalent to a capacitanceelement, and the switch element SW4 and the inductor element L1 form aparallel resonance circuit. With sharp attenuation characteristics nearthe first cut-off frequency by parallel resonance, the low pass filter150 may attenuate an intermodulation distortion signal. In the secondoperation mode, the switch element SW4 is in an ON state such that thelow pass filter 150 has a second cut-off frequency that is higher thanthe first cut-off frequency. When the switch element SW4 is in the ONstate, the switch element SW4 behaves as equivalent to a resistanceelement, and the switch element SW4 and the inductor element L1 do notform a parallel resonance circuit. Accordingly, attenuationcharacteristics of the low pass filter 140 become moderate, and the passband of the low pass filter 140 extends from a low frequency band to awide region including a low frequency band and a high frequency band.Therefore, insertion loss by the low pass filter 150 occurring in an RFsignal output from the power amplifier 51 may be reduced.

The first cut-off frequency of the low pass filter 150 is determinedbased on element values (capacitance values and inductance values) ofthe individual circuit elements (the capacitance elements C21 and C31and the inductor element L1) forming the low pass filter 140 and an OFFcapacitance of the switch element SW4. The second cut-off frequency ofthe low pass filter 150 is determined based on the element values(capacitance values and inductance values) of the individual circuitelements (the capacitance elements C21 and C31 and the inductor elementL1) forming the low pass filter 150 and an ON resistance of the switchelement SW4.

Next, a circuit configuration of a low pass filter 160 according to aseventh embodiment of the present disclosure will be described withreference to FIG. 10, focusing on differences from the low pass filter150. The low pass filter 160 has additional lumped elements based on thelow pass filter 150. The additional lumped elements are capacitorelement C1′, C4 and C4′ and switch element SW4 and SW5. Each switchelement SW4, SW5 is a semiconductor switch, such as a field effecttransistor, which is called an SPST switch, and performs ON/OFFoperations in response to, for example, a control signal from thebaseband IC 20 or the RFIC 30. The switch element SW4 and the capacitorelement C1′ are connected to each other in series, and together inparallel with the inductor element L1 and the capacitor element C1. Theinductor element L1 and the capacitor element C1 are connected in serieswith a signal line for the low pass filter 160. The switch element SW5and the capacitor element C4′ are connected to each other in series, andtogether in parallel with the inductor element L4 and the capacitorelement C4. The inductor element L4 and the capacitor element C4 areconnected in series with a signal line for the low pass filter 160.

It is noted that the foregoing embodiments are provided to facilitateunderstanding of the present invention and are not intended to limit thescope of the present invention. Changes and improvements may be made tothe present invention without departing from the scope of the presentinvention, and the present invention also includes equivalents thereof.That is, design changes may be made to the embodiments in an appropriatemanner by those skilled in the art, and such embodiments are also withinthe scope of the present invention as long as they have features of thepresent invention. The individual circuit elements included in theembodiments and the arrangements thereof are not limited to thosedescribed above as examples, and they may be changed in an appropriatemanner. For example, a state in which “a circuit element A is connectedto a circuit element B” represents the case where a signal path may beselectively established via a circuit element C (for example, a switchelement) between the circuit element A and the circuit element B as wellas the case where the circuit element A is directly connected to thecircuit element B. In addition, the circuit elements included in theembodiments can be combined with each other as long as it is technicallypossible, and such combination is also within the scope of the presentinvention as long as the combination has features of the presentinvention.

While preferred embodiments of the invention have been described above,it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the invention. The scope of the invention, therefore, isto be determined solely by the following claims.

What is claimed is:
 1. A low pass filter that receives and outputs atleast one radio frequency (RF) signal, comprising: a first switch; afirst capacitance element having a first end connected to a signal lineof the low pass filter and having a second end connected to ground; anda second capacitance element having a first end connected to the signalline and having a second end connected to ground via the first switch,wherein when a plurality of RF signals having different frequency bandsare output at the same time by carrier aggregation, the first switch isclosed such that the low pass filter has a first cut-off frequency thatis lower than a frequency of an intermodulation distortion signalgenerated by the carrier aggregation, and such that the firstcapacitance element and the second capacitance element are connected inparallel between the signal line and ground, and wherein when a singleRF signal is output, the first switch is open such that the low passfilter has a second cut-off frequency that is higher than the firstcut-off frequency, and such that the second capacitance element is notconnected to ground.
 2. The low pass filter according to claim 1,further comprising: an inductor element that is connected in the signalline; and a second switch that is connected in parallel with theinductor element, wherein when a plurality of RF signals havingdifferent frequency bands are output at the same time by the carrieraggregation, the second switch is open such that the low pass filter hasthe first cut-off frequency, and wherein when a single RF signal of afrequency band is output, the second switch is closed such that the lowpass filter has the second cut-off frequency.
 3. The low pass filteraccording to claim 1, wherein the first cut-off frequency is based onvalues of the first and second capacitance elements and a resistance ofthe first switch when closed, and wherein the second cut-off frequencyis based on values of the first and second capacitance elements and acapacitance of the first switch when open.
 4. The low pass filteraccording to claim 2, wherein the first cut-off frequency is based onvalues of the first and second capacitance elements, the inductorelement, a resistance of the first switch when closed, and a capacitanceof the second switch when open, and wherein the second cut-off frequencyis based on values of the first and second capacitance elements, theinductor element, a capacitance of the first switch when open, and aresistance of the second switch when closed.
 5. The low pass filteraccording to claim 1, wherein the low pass filter is connected to anoutput of a power amplifier or to an input of a low-noise amplifier. 6.The low pass filter according to claim 2, wherein the low pass filter isconnected to an output of a power amplifier or to an input of alow-noise amplifier.
 7. The low pass filter according to claim 3,wherein the low pass filter is connected to an output of a poweramplifier or to an input of a low-noise amplifier.
 8. The low passfilter according to claim 4, wherein the low pass filter is connected toan output of a power amplifier or to an input of a low-noise amplifier.9. The low pass filter according to claim 1, further comprising a firstparallel connection of an inductor element and a capacitance element,the first parallel connection being in the signal line.
 10. The low passfilter according to claim 9, further comprising an inductor elementconnected in the signal line in series with the first parallelconnection, wherein the first end of the first capacitance element andthe first end of the second capacitance element are connected to thesignal line at a node between the inductor element and the firstparallel connection.
 11. The low pass filter according to claim 9,further comprising a second parallel connection of an inductor elementand a capacitance element, the second parallel connection being in thesignal line in series with the first parallel connection, wherein thefirst end of the first capacitance element and the first end of thesecond capacitance element are connected to the signal line at a nodebetween the first parallel connection and the second parallelconnection.
 12. The low pass filter according to claim 1, wherein thefirst end of the first capacitance element and the first end of thesecond capacitance element are connected to each other at a node, andthe low pass filter further comprises an inductor element connectedbetween the signal line and the node.